This application claims priority from Korean Patent Application No. 2003-83359, filed on Nov. 22, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a dispersion-shifted optical fiber for an optical parametric amplifier.
2. Description of the Related Art
As the application fields of optical parametric amplifiers (OPAs) are broadened, interest and research on the OPAs are increasing. The OPAs can amplify in all wavelength regions using an optical fiber as a gain medium and have wide gain bandwidth. Furthermore, the OPAs have a low noise figure and a high conversion efficiency because an input signal at a frequency ωs and an idler signal, which is generated at the frequency ωl=2ωp−ωs by four-wave mixing (FWM) in a nonlinear medium, can simultaneously be amplified by only one pump signal at a frequency ωp. Therefore, the OPAs can be used not only as S- and U-band amplifiers but also as wavelength converters, switches, and signal processing.
To maximize the FWM efficiency and to enhance the performance of such OPAs, a pump with an output of 2 watt or more and an optical fiber with nonlinear coefficient as high as 20 W−km−1 or more are required. At an early stage, a conventional dispersion-shifted fiber (DSF) was used as nonlinear gain medium for the OPA.
The conventional dispersion-shifted fiber was designed to minimize the transmission loss and the dispersion slope for long-haul lightwave system. Therefore, the conventional dispersion-shifted fiber has a low loss but its nonlinear coefficient is as very low as 2 W−km−1. Furthermore, dispersion-shifted fibers have been developed toward broadening an effective area and decreasing a bending loss.
As shown in FIGS. 1A, 1B, and 2, dispersion-shifted fibers with wide effective area were designed to have a graded or trapezoidal refractive index core (U.S. Pat. No. 6,535,676B1, filed by Louis-Anne de Montmorillon, titled “Optical Fiber with Optimized Ratio of Effective Area to Dispersion Scope for Optical Transmission System with Wavelength Multiplexing”, and published on Mar. 18, 2003) and/or a dual clad (U.S. Pat. No. 6,546,177 B1, filed by Shoichiro Matsuo, titled “Dispersion Shifted Optical Fiber”, and published on Apr. 8, 2003). In particular, FIG. 2 shows a refractive index profile of a dispersion-shifted fiber with a ring in an outer clad to decrease a bending loss (U.S. Pat. No. 6,424,775 B1, filed by Mariamme Paillot, titled “Single Mode Dispersion-Shifted Optical Fiber Comprising an External Refractive Index Ring”, and published on Jul. 23, 2002).
In recent years, it has been reported that heavy metals such as lead and bismuth increase the nonlinear refractive index of optical fibers (M. Asobe, et al., “Laser-diode-driven ultrafast all-optical switching by using highly nonlinear charcogenide glass fiber”, Opt. Lett., 18, 1056–1058 (1993)).
U.S. Pat. No. 5,148,510, filed by Nicholas F. Borrelli, et al., titled “Optical fiber made of galliobismuthate glasses and optical devices using the same” discloses that the susceptibility of heavy metal oxide glasses is directly proportional to the concentration of heavy metals. A photonic crystal fiber (PCF) has been actively studied as a highly nonlinear fiber (U.S. Pat. No. 6,243,522 B1, filed by Douglas Clippinger Allan, et al., titled “Photonic crystal fiber”, and published on Jun. 5, 2001). The PCF is made by inserting a plurality of glass capillaries into a glass tube, which enables sufficient reduction of an effective area.
In view of these advantages, the PCF has been used for an OPA. However, the PCF has disadvantages such as large loss and laborious splicing problem between it and a normal silica optical fiber. Unlike a manufacturing process of a normal silica optical fiber that includes deposition by chemical reactions of various materials such as SiCl4, an exposure of the capillaries to an outer environment in a manufacturing process of the PCF is indispensable due to the stacking of capillaries. For this reason, impurities may be introduced into the capillaries during the exposure of the capillaries to an outer environment and then a loss of the PCF is increased. Furthermore, several air voids present on a cross-section of the PCF induces a large refractive index difference between the PCF and a normal silica optical fiber, which renders the splicing between the PCF and the silica optical fiber difficult.